A messenger Bag is a remarkably versatile item to make, this design can be adapted to almost any size to fit your purpose.
I focused on an iPad, but you can choose your own use.
A simple C Shape Gusset was employed along with a Loop and Strap Closure.
This is a simple enough build for those early in their leather working journey to approach, yet contains enough information for an adept to manipulate to their own design.